1. Field of the Invention
The present invention relates generally to electro-optically active waveguide segments, and more particularly to the use of a selective voltage input to control the phase, frequency and/or amplitude of a propagating wave in the waveguide.
This invention is made with government support under AFSOR grant number F49620-96- 1-0079, awarded by the United States Department of Defense. The government may have certain rights in this invention.
2. Description of the Prior Art
Modulation and switching of optical signals are basic functions in an optical communication system. Through modulation, the information to be communicated is expressed in one or more parameters of a light signal, such as the amplitude, the polarization, the phase or frequency of the field, or of the magnitude or spatial distribution of the power and/or intensity. Through switching, the light signal may be routed through a network of optical nodes and connections. Optical connections are mostly realized with the help of glass fibers.
Moreover, standard glass fibers are not polarization-retaining, whereas many optical devices, such as receivers, switches, and modulators, are polarization sensitive. With a view towards low cost mass production, precisely integrated embodiments are of great importance for large-scale introduction such as, e.g., in optical communication and/or distribution networks with large numbers of connections. Therefore, a cheap, stable and electrically controllable polarization control devices are needed to satisfy a long felt need in the communications industry.
Several devices have been disclosed that provide phase control. For example, U.S. Pat. No. 5,239,407, by Brueck et al, discloses the use of thermal poling in a waveguide to establish a preferred non-linearity. One significant drawback to this approach is that this non-linearity may only be achieved along one axis. Additionally, the non-linearity is generated only on one side of the sample, e.g., the positive biased side of the sample and therefore does not effect a wavefront in a consistent manner across the wavefront. Finally, to avoid breakdown, a significant portion of the cladding layer is removed and thus the optical loss in this device is significant.
Fujiwara et al. discloses, in an article entitled "UV Excited Poling and Electrically Tunable Bragg Gratings in a Germanosilicate Fiber," two innovations: 1) the use of an ultraviolet (UV) beam in combination with an applied electric field to produce poling; and 2) the use of two internal electrodes for applying a voltage across a Bragg grating. Their technique, however, has a number of drawbacks. Specifically, the fiber is drawn from a preform with two holes for electrode wires that are to be inserted following the fiber drawing. This wire insertion is a difficult manufacturing step. To avoid breakdown, one wire is inserted from each end of the fiber. This means that the modulation frequency is limited to low values since a high-speed travelling wave geometry is not possible. Furthermore, splicing to either end of the fiber is not possible because of the electrodes. Thus, discrete optical system alignment for coupling into the fibers would be necessary and would negate any benefit from an electro-optically active fiber segment. Additionally, the use of UV poling as taught by Fujiwara et al. has the added disadvantage of requiring custom fabrication of a preform that are relatively difficult to fabricate.
Finally, U.S. Pat. No. 5,617,499, by Brueck et al., discloses a poled electro-optic fiber segment as illustrated in FIG. 1 of the present application. The device comprises a first electrical contact 1, a fiber core 2, a cladding 3, and a second electrical contact 4. As may be seen, a significant portion of cladding 3 has been removed. Thus, the optical loss in this device may be significant. Additionally, the modulation frequency is limited to low values since a high-speed travelling wave geometry is significantly restricted. Finally, the Brueck et al. device is limited to temperature/electric poling because access to core 2 via cladding 3 is impaired by first electrical contact 1 and the bottom electrical contact 4 making it difficult to perform UV poling in such a structure. Further, poling using elevated temperatures may significantly degrade the reflectivity of a Bragg grating disposed in the poled waveguide.
Therefore, there needs to be an easily manufactured poled electro-optic fiber segment which allows for poling by: 1) voltage, 2) ultraviolet radiation, 3) thermal heating, or 4) any combination of the above.
The use of tunable Bragg gratings is known in the prior art. These gratings have been controlled by thermal tuning, piezoelectric tuning, Fabry-Perot tuning, and refractive index tuning in bulk semiconductors. Each of these methods of tuning has disadvantages associated with them. In particular, the speed of tuning is in the range of a few milliseconds for thermal tuning and a few jsec for piezoelectric tuning. Additionally, such methods have the disadvantage of applying stress on an optical fiber that can reduce the strength of the fiber over a period of time. Finally, these methods tend to be very bulky. On the other hand, integration of bulk non-linear materials such as lithium niabate to a fiber segment is lossy, expensive and is not desirable. Therefore, other methods of tuning a Bragg grating are needed.
The first optical grating or so-called Bragg grating was made in 1978 using the standing wave pattern originating from two counter-propagating beams in a Ge-doped core optical fiber. Since that time, techniques have been developed to exploit the photosensitivity of germanosilicate fibers, the photosensitivity being established by the bleaching of oxygen deficient centers by UV light to create the regions of differing refractive index. The refractive index change, which is induced by the UV light, arises from the creation of polarizable color centers and structural rearrangement of the glass network.
While these gratings have evolved, an efficient control system for this type of grating has not yet been demonstrated. Several attempts to develop such as control system have fallen short of the goal of a commercially usable device. For example, U.S. Pat. No. 5,617,499, by Brueck et al, discloses the use of a Bragg grating in a poled device, but provides no teaching of control. Additionally, Fujiwara et al. discloses the use of a Bragg grating in their device, but provides no teaching of control. Unfortunately, both these devices have the drawbacks discussed above.
Therefore, there needs to be an easily manufactured electro-optic fiber segment having a tunable Bragg Grating which reduces the possibly of breakdown during poling while achieving a smaller dimmension between electrodes without producing a high loss for the mode of propagation in the core region or without degrading the Bragg grating.